This study responds to RFA-AR-10-011 on Ancillary Studies to Large Ongoing Clinical Projects (R21) by conducting a time-sensitive study to enhance the scientific content of a large clinical study, the Osteoarthritis (OAI) Physical Activity Cohort. Knee osteoarthritis (OA) ranks with cardiac disease as the most disabling chronic diseases in older adults. Physical activity offers a promising approach to improve the quality of life for persons with knee OA However, evidence based research on the role of physical activity in to improve the health of persons with knee OA lags behind research in the general population. One problem is the lack of validated procedures to translate objective accelerometer data into physical activity measures. This aims of this study address this issue by releasing valid objective physical activity measures to the OAI public data set obtained from accelerometer monitoring. Aim 1. Develop and refine methodology to verify an algorithm in a knee OA population to translate accelerometer data into valid physical activity measures. Aim 2. Develop, formulate, document, and release to the OAI public data set a robust set of physical activity measures from accelerometer recordings on the OAI Physical Activity Cohort. Products from this study include the following: "Methodology work on a data driven approach to convert accelerometer data into valid estimates of physical activity behavior in the knee OA disease population; "Public access to the documented accelerometer algorithm validated for knee OA;"Public access to a robust set of processed and validated objective physical activity measures on OAI physical activity cohort "Documentation of the publically released physical activity measures written 1) for the general user and 2) for the technical user. The study creates a data driven approach to convert accelerometer data into valid estimates of physical activity behavior in the knee OA disease population. The results of from this study will provide a benchmark approach to translate accelerometer data into meaningful measures of physical activity for the entire osteoarthritis research community. In addition, this study will also release a robust set of objective physical activity d to the OAI public data. Valid physical activity data from the OAI Physical Activity Cohort added to the OAI public clinical and imaging data will create the largest available data set with objectively measured physical activity on persons with knee OA. Taken together, the two parts of this study will accelerate evidence-based physical activity research in knee OA population.